This information is from the RSPCA Knowledge Base:

The main reason for desexing is to prevent unwanted pregnancies. The RSPCA takes in 160,000 animals every year and many of these are the result of unplanned breeding. Regretfully, many have to be euthanased because there are not enough suitable homes for them to go to. Desexing can help prevent and reduce this.

Desexed animals are generally less likely to get diseases and illness such as mammary cancer and uterine infections in females and cancer and prostate problems in males.Desexing commonly reduces behaviour problems such as roaming, aggression and urine marking in males. In females it prevents mating behaviour and false pregnancy.

The RSPCA practises early age desexing from the age of eight weeks when the surgery is simple and recovery is rapid. Early age desexing is an effective way of reducing accidental pregnancy in young animals and ensuring compliance with local council desexing requirements. If your puppy or kitten was not desexed prior to sale, they must be desexed before they are able to produce any unintended litters. There is absolutely no benefit in letting females have one litter before they are desexed.

Traditionally, vets have recommended that cats and dogs are desexed between 5½ and 6 months of age. But over the past decade, desexing at an earlier age (from eight weeks onwards) has become more common. This is known as early-age dexing or EAD. The RSPCA has been desexing kittens and puppies in its shelters at this earlier age for many years, and based on this experience and the cumulation of considerable scientific evidence, the RSPCA considers EAD to be a safe and effective strategy for the wider community to prevent unintended litters in cats and dogs.

In addition to helping to prevent unwanted pregnancy, the EAD can offer significant animal welfare benefits when compared to traditional age desexing. Desexing surgery is faster and easier when carried out on younger patients as their anatomical structures are less developed. There is less tissue trauma and less tissue handling involved, the surgery incision site is smaller, and bleeding is reduced and minimal. It also takes less time to prepare the animals for EAD surgery which means less time under general anaesthesia. The anaesthetic recovery and wound healing times are also shorter, providing further animal welfare benefits. It also significantly reduces the risk of mammary cancer in both dogs and cats. These benefits are in addition to all of the commonly accepted benefits associated with desexing, such as a reduction in wandering/roaming and undesirable sexual behaviours such as mounting and urine spraying.